August 17, 2007

Climate scientists have known for many years that the energy output from the Sun varies and believe it or not, when the Sun is putting out more energy, the Earth heats up and when the Sun cools down, so does the Earth. What appears to be so simple is actually much more complex as the Sun can vary its output differentially in the various portions of the electromagnetic spectrum. For example, the Sun can actually increase its production of gamma rays while decreasing the level of infrared emission, and these patterns of energy output can reveal themselves quite differently in terms of response of Earth’s climate.

Nonetheless, over the past century, the correlation between solar irradiance (in Watts per square meter) and the planetary temperature anomaly (in °C) is highly statistically significant; the Pearson product-moment correlation coefficient is 0.51 and is beyond the 0.99 level of statistical confidence (see below). The line on the plot basically represents a simple statistical linear “model” that predicts the planetary temperature anomaly given a specific output level of the Sun. For every year we could examine the difference between the observed temperature anomaly and the predicted anomaly given solar output – the difference (observed – predicted) is called the residual which should have a mean of zero over the study period. The residuals should fluctuate from year to year, and they should behave in a random fashion.

Well, the residuals average zero over the 1910-2003 period, but they are anything but random (see below). Sometime in the mid-1970s, the linkage between solar output and Earth’s temperature weakened and a warming appears in the temperature record unexplained by variations or trends in solar output. This pattern has been noted my many scientists, and to say the least, it is popular to suggest that the radiative effect of the build-up of greenhouse gases finally overwhelmed the climate system and forced the temperature of the Earth to climb. According to some, we finally entered the climatic anthropocene in the 1970s – the greenhouse signal is viewed as loud and clear in the global climatic record.

Time series plot of residuals (°C) from the regression line over the period 1910-2003 (from Balling and Sen Roy, 2005)

An article has appeared in Geophysical Research Letters that is sure to ruffle the feathers of the greenhouse advocates. The piece is by Anastasios Tsonis and two associates at the University of Wisconsin-Milwaukee; the work was funded by the National Science Foundation, the Department of Energy, and NASA.

Tsonis et al. examined the statistical behavior of four major climate indices over the period 1910-2000 including the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO), the El Niño/Southern Oscillation (ENSO), and the North Pacific Oscillation (NPO). They write “These indices represent regional but dominant modes of climate variability, with time scales ranging from months to decades. NAO and NPO are the leading modes of surface pressure variability in northern Atlantic and Pacific Oceans, respectively, the PDO is the leading mode of SST variability in the northern Pacific and ENSO is a major signal in the tropics. Together these four modes capture the essence of climate variability in the northern hemisphere.”

Understand that this team comes from a Department of Mathematical Sciences and they tend to decompose the behavior of these four indices into mathematical functions that describe oscillations and persistence qualities of the four time series. Tsonis et al. develop a measure of synchronization of the system and they note “Synchronization between nonlinear (chaotic) oscillators occurs when their corresponding signals converge to a common, albeit irregular, signal”. OK – they have Ph.D.’s in mathematics, we don’t, so let’s get to the bottom line.

Tsonis et al. write “We find that in those cases where the synchronous state was followed by a steady increase in the coupling strength between the indices, the synchronous state was destroyed, after which a new climate state emerged. These shifts are associated with significant changes in global temperature trend and in ENSO variability. The latest such event is known as the great climate shift of the 1970s.” Two other times between 1900 and 2000 had the same alignment when synchronization was following by an increase in coupling strength. One occurred ~1912, and the Earth warmed steadily for the next 30 years, while the other occurred in ~1941, and the temperature of the Earth remained remarkably steady for a 30 year period. Furthermore, Tsonis et al. ran a global climate model over a 100 year period and found that each time synchronization was followed by an increase in coupling strength, the Earth’s climate seemed to move into a new state defined by warming or cooling.

Tsonis et al. end their article noting that while “The standard explanation for the post 1970s warming is that the radiative effect of greenhouse gases overcame shortwave reflection effects due to aerosols,” their result “suggests an alternative hypothesis, namely that the climate shifted after the 1970s event to a different state of a warmer climate, which may be superimposed on an anthropogenic warming trend.”

The Tsonis et al. conclusions allow us all to speculate that some degree of post-1970s warming may have been in the cards with no greenhouse forcing whatsoever.